The pimary goal of this research project is to evaluate the functional role of the auditory thalamo-cortical system in hearing behavior in primates. A joint approach including anatomical degeneration studies of auditory thalamic projections to the cortex, animal psychophysical procedures, and chronic electrophysiological techniques will be employed. The location, extent and organization of projections from the medial geniculate to auditory cortex will be examined. Hearing behavior will be evaluated in a simple reaction time task which has been demonstrated to yield precise measures of threshold and suprathreshold hearing. Unit activity will be recorded from higher central auditory structures, particularly auditory cortex, simultaneously with the evaluation of hearing performance. We will attempt to describe the response chracteristics of auditory cortex activity in relationship to: (1) controlled auditory stimulation, (2) structural features of the tissue, (3) physiological chracteristics of the input to this tissue from medial geniculate and inferior colliculus (4) selected behavioral conditions, and (5) experimental (pathological) changes in auditory system function. The research is designed to obtain information which will further our understanding of the role of central auditory structures of hearing and hearing dysfunction.